The problem of providing fast and uniform cooling of autoclave furnaces has been addressed repeatedly in the prior art for the reason that it is such a desirable objective. See, for example, U.S. Pat. Nos. 3,940,245; 4,022,446; 4,235,592; 4,246,957; 4,280,807; and 4,131,419.
Simply opening convection paths for the pressurized fluids along the interior wall of the pressure vessel increases the cooling rate but this approach has two decided drawbacks: the temperature distribution along the axis of the workpiece is often unacceptable since flow is entirely dependent upon the "buoyancy effect." Also, the fluids carried against the interior wall of the pressure vessel may be at the maximum vessel temperature when cooling commences. Notwithstanding the vessel walls are liquid cooled, the contact with pressurized fluids at the maximum temperature can be damaging to the vessel. Forced circulation during cooling within the workspace, that is, around the workpiece or workpieces, improves the temperature distribution along the axis of the workpiece but still allows the hottest pressurized fluids in the vessel to flow against the vessel wall.
It is an object of this invention to provide a rapid cooling autoclave furnace wherein multiple circulating flows of pressurized fluids within the vessel minimize the temperature differentials of the circulating fluids as they pass axially along the workpiece and to reduce the maximum temperature of the fluids along the inner wall of the pressurized vessel.
It should be understood that there are two types of temperature nonuniformity within the workspace or workpieces that are undesirable in an autoclave furnace. The first is the nonuniformity of the workspace or the workpieces along the vertical axis of the workspace. This is a result of the tendency of hotter fluids to rise in the vessel. This results in uneven thermal gradients from top to bottom in the furnace and workpieces. The second type of temperature nonuniformity is within the workpieces themselves. Due to heating or cooling of the surface of the workpieces faster than heat can be transferred into or out of the interior of the workpieces an inside-outside temperature gradient may develop.
The forced circulation of fluids within the workspace, as is taught in the prior art, helps to overcome the temperature gradient along the vertical axis of the workspace. It does so by redistributing the fluids so that the natural rise of the hotter fluids is controlled and directed. However, forced circulation can theoretically increase the inside-outside temperature gradients by increasing heating or cooling rates. It is an inescapable fact that the faster the heating or cooling of the workpieces, the greater the inside-outside temperature gradients. The challenge is to use circulation in the workspace in a way to promote top to bottom uniformity but also in a way not to cause undesirable inside-outside nonuniformity.
It is an object of applicants' invention to provide two circulating flows of pressurized fluid, one passing down along the inner wall of the pressure vessel for cooling the mass of fluid in that flow path and another passing down through the workspace to pick up heat from the workpiece or workpieces during cooling.